Charcot-Marie-Tooth Disease (CMT)
What is it?
Charcot-Marie-Tooth Disease (CMT) is an inherited disorder that affects the nerves that carry information to and from the spinal cord. It causes symptoms of weakness and loss of sensation in the limbs. CMT is named for the three neurologists who first described it, Jean Martin Charcot, Pierre Marie, and Howard Henry Tooth. It is also known as hereditary motor and sensory neuropathy, and peroneal muscular atrophy, because the peroneal muscles in the lower legs are often affected in the early stages of the disease.
Who gets it?
Charcot-Marie-Tooth Disease (CMT) is a hereditary neuropathy, which means it is a disorder of the nervous system passed from parent to child. About 1 in 2,500 people in the United States has a form of CMT. CMT is the most common hereditary
neuropathy.
What causes it?
CMT affects the groups of nerve cells that carry information to and from the spinal cord, called the peripheral nerves. The peripheral nerves have cells in the brain and spinal cord and nerve fibers that run from the brain and spinal cord to the whole body. The signals the peripheral nerves send control muscle movement, and the signals they receive relay feelings of pain, pressure, temperature and position back to the brain and spinal cord. The genetic defect that causes CMT affects the myelin sheath, which is the coating that insulates the nerve fibers so nerve impulses can travel freely over the fibers. If the myelin is defective, the impulses either travel more slowly or carry a weaker signal. The areas of the body most affected are those farthest from the spinal cord, the hands and feet. Most forms of CMT are inherited as an autosomal dominant trait, which means that a child need inherit only one abnormal gene to have the disease. The chance of a child inheriting this one abnormal gene is 50 percent with each conception. Scientists have identified three chromosomes that, when defective, result in some form of CMT. They are chromosome 1, chromosome 17, and chromosome X. Defects in chromosomes 1 and 17 affect the myelin sheath and cause the nerve impulses to travel more slowly than is normal. Defects in chromosome X are a little different. These defects may affect the myelin sheath and nerve fibers themselves. They are inherited as an X-linked trait, which means the gene is carried on the X chromosome. In X-linked dominant conditions, males and females need only one abnormal X chromosome gene to show symptoms of the disease. Females with X-linked CMT may have milder symptoms than men. This is because women have two X chromosomes and the normal chromosome can serve as backup for the abnormal one. Men have one X and one Y chromosome. Because of this, men can only pass this defective gene onto their daughters, not their sons, who receive the normal Y chromosome. A less common type of CMT, called CMT type 3, or Dejerine-Sottas disease, is inherited as an autosomal recessive trait. This means both parents need to contribute a defective gene for the child to have symptoms of the disease.
What are the symptoms?
CMT is usually divided into three types, determined by the way in which the peripheral nerves are affected and the symptoms caused. Most people (around two-thirds) have type 1 CMT, which affects the myelin sheath. Symptoms of type 1 usually appear during late childhood or adolescence and include weakness in the lower legs. This weakness causes a condition known as foot drop, where the foot tends to flop down, and loss of muscle tone in the calves. The child may have difficulty walking and running because of foot deformities such as high arches and toes that curl under (clawed toes). Because of this and foot drop, he or she may trip over curbs, have to take higher than normal steps, and walk slowly and carefully. As the disease progresses, the muscle weakness affects the hands, and the child experiences mild to severe loss of sensation in the hands and feet. Tremor, or shaking, may also occur. The remaining one-third of patients have type 2 CMT, which affects the nerve fibers. Because it progresses more slowly, type 2 is usually diagnosed a little later in life than type 1. Symptoms are similar to type 1. In types 1 and 2 CMT, the disease progresses slowly and doesn't affect life expectancy. The most severe form of CMT is type 3, also known as Dejerine-Sottas disease. Children with type 3 CMT may not develop early motor skills such as walking until they are three or four years old. Other symptoms include severe sensory problems, hearing loss, and the loss of the ability to walk during adulthood.
How is it diagnosed?
Your doctor will diagnose CMT by taking a complete medical history and performing a thorough neurological exam. He or she will test the child's reflexes and look for any shriveling or wasting away (called atrophy) of the muscles. The doctor will test the strength of the muscles by asking the child to push or pull against resistance. He or she will test whether the child can distinguish between different sensations by gently applying pressure, heat, and vibration to various areas of the hands and legs. A test called electromyography is used to check how the nerves are functioning. Small needles are inserted into a muscle to record the muscles electrical activity. The electrical activity will be abnormal if there is damage to the peripheral nerves. The doctor may also perform similar tests called nerve conduction studies, or nerve conduction velocity tests, which measure the speed at which the motor nerves conduct impulses. With this test, a small electrical charge is used to stimulate a motor nerve. When the impulse reaches the muscle, it should contract. By measuring the time it takes the impulse to reach the muscle as well as the amount of impulse that reaches the muscle, your doctor can diagnose CMT type 1 and 2. These procedures can cause some minor discomfort, but may be necessary to make an accurate diagnosis. Occasionally, doctors may remove a small piece of the nerve, called a nerve biopsy, to look for the swelling that is usually seen with CMT type 1. DNA testing is available for CMT type 1. DNA testing is usually used to identify people at risk for the disease, and for family planning purposes where there is a family history of the disease.
What is the treatment?
While there is no treatment to prevent this disease from getting worse, there are ways to reduce the physical symptoms that cause problems with movement and coordination Physical therapy can help keep muscles more flexible by using range of motion and balance exercises. Stretching the muscles can also help reduce muscle shortening, called contracture, which often occurs with this disease. Some patients wear lightweight braces to correct foot drop. Occupational therapy is also recommended to help the patient perform daily activities such as getting dressed, eating, opening doors and bottles, and writing. If tremor is a problem, the patient should avoid caffeine and alcohol. In severe cases, drugs called beta-blockers can help reduce tremor. Certain drug combinations can also cause muscle damage, so it is important that your doctors and pharmacist are aware of your condition. Surgery is sometimes used to correct severe foot deformities.
Self-care tips
There is no way to prevent CMT in people who have inherited the gene or genes that cause this disease. If your family has a history of this disease, you may want to undergo genetic testing for family planning purposes. If you or your child have this disease, education is an important tool in treatment. Learn all you can about the disease and its treatment, and develop a good support system so you or your child can live life to the fullest.
This information has been designed as a comprehensive and quick reference
guide written by our health care reviewers. The health information written
by our authors is intended to be a supplement to the care provided by your
physician. It is not intended nor implied to be a substitute for
professional medical advice.
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